1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor substrate and a method of fabricating a light emitting device. More particularly, the present invention relates to a method of fabricating a semiconductor substrate and a method of fabricating a light emitting device based on separating a growth substrate.
2. Discussion of the Background
Light emitting diodes (LEDs) that have a gallium nitride (GaN) based semiconductor may be used for various applications, such as traffic signals, backlights in liquid crystal panels, and the like. It is known that luminous efficiency of an LED is influenced by the dislocation density of crystals and crystal defects. GaN-based semiconductor crystals may be grown on a heterogeneous substrate, such as sapphire or the like. However, lattice mismatch or a difference of thermal expansion coefficients may occur between a GaN layer and a substrate, which may result in a high dislocation density or an increase in defects.
The GaN-based semiconductor crystals may be grown on a homogeneous substrate, such as a GaN substrate and the like. However, it may be difficult to form a GaN melt and to fabricate a GaN substrate due to a high dissociation rate of nitrogen in GaN, and the like. A GaN bulk crystal grown for a GaN substrate may be separated by mechanical grinding or laser ablation. However, it may be difficult to produce a GaN substrate of a practical size using these methods. Particularly, the laser ablation method may require a considerable amount of time to perform, which may increase the cost of a GaN substrate.
GaN crystal growth is shown and described in “Polycrystalline GaN for light emitter and field electron emitter applications,” by S. Hasegawa, S, Nishida, T. Yamashita, H. Asahi, (Thin Solid Films 487 (2005), pp 260-267) (hereinafter, “Hasegawa, et al.”), which is hereby incorporated by reference in its entirety for all purposes as if fully set forth herein. In Hasegawa, et al., GaN crystals are grown on quartz substrates, high-melting-point metal is substrates of Tungsten (W), Molybdenum (Mo), Tantalum (Ta) and Niobium (Nb), and Silicon (Si) substrates using plasma assisted molecular beam epitaxy, respectively.
Since it may be difficult and costly to fabricate the GaN substrate, semiconductor devices such as LEDs or laser diodes are generally manufactured by growing a GaN layer on a heterogeneous substrate such as sapphire and the like. However, as mentioned above, the high dislocation density or the increase in defect density may degrade the luminous efficiency of the LED. In addition, the sapphire substrate has a lower thermal conductivity than the GaN substrate, and may cause deterioration in heat dissipation properties of a device. Thus, use of a sapphire substrate for LEDs or laser diodes may limit operational lifespan thereof.
To solve such problems of the sapphire substrate, a laser lift-off process has been developed in which a secondary substrate is attached to a GaN layer after the GaN layer is grown on a heterogeneous substrate as a growth substrate. The secondary substrate in turn is locally decomposed from an interface between the sapphire substrate and the GaN layer by an excimer laser to remove the sapphire substrate. This process is used to fabricate a vertical-type light emitting device suitable for large size LEDs (i.e., a power chip) and the like.
As described above, however, separation of the growth substrate using the laser may require an extended processing time, thereby causing an increase in manufacturing costs of the light emitting device. Further, since it may be necessary to increase transmittance of the sapphire substrate to laser beams in order to radiate a laser beam through the sapphire substrate, it may be necessary to polish an exposed surface of the sapphire substrate, which may decrease a thickness of the sapphire substrate.